The present invention relates to binding, rendering insoluble, and precipitating a wide range of heavy metals, and more specifically to compounds and methods for multidentate binding or chelation of heavy metals utilizing a novel class of sulfur-containing ligands.
Heavy metal pollution is an existing and growing worldwide problem. During the past few decades, federal and state governments have instituted environmental regulations to protect the quality of surface and ground water from heavy metals. In response to these regulatory requirements, numerous products have been developed to precipitate heavy metals from surface and ground water, and soil. An example of a reagent for precipitating divalent and univalent heavy metals from water is TMT-55, or 2,4,6-trimercaptotriazine, trisodium salt nonahydrate, manufactured by Degussa Corporation USA. The mode of action, chemistry, and stability of resulting heavy metal-trimercaptotriazine precipitates is unknown.
There are numerous industrial and environmental situations where ligands capable of binding metals are utilized for remediation purposes. For example, waste water issuing from waste treatment facilities, from the chlor-alkali industry, from the metal finishing industry, and from certain municipal landfills often presents a metal contamination problem. Similarly, the metal content of water exiting both functional and abandoned mines is a significant environmental issue in geographical areas with a heavy mining industry. Soils located in areas near natural gas pumphouses suffer a similar metal contamination problem. Thus, there is a need in the art for ligands capable of binding and precipitating heavy metals from both aqueous solutions and solid substrates such as soil.
It is known in the art to use sulfur-containing compounds to bind heavy metals. For example, Thio-Red(copyright) is a chemical reagent used for precipitating divalent heavy metals from water. This product is a complex aqueous solution of sodium (with or without potassium) thiocarbonate, sulfides, and other sulfur species. Thio-Red(copyright) ultimately removes Cu, Hg, Pb, and Zn from aqueous solutions through the formation of metal sulfides (i.e. CuS, HgS, PbS, and ZnS) rather than metal thiocarbonates. Sodium and potassium dialkyldithiocarbamates are also widely used as metal precipitants. However, the limited array of binding sites for heavy metals is a major concern with most ligands presently used on a commercial basis for heavy metal ion precipitation. Ligands that lack sufficient binding sites may produce metal precipitates that are unstable over time and under certain pH conditions. Such unstable precipitates may release bound metal back into the environment, thereby proving unsatisfactory as treatment or remediation agents. Further, these compounds may form simple metal sulfides which bacteria are capable of methylating (in the case of Hg, forming the water-soluble and highly toxic cation MeHg+. Accordingly, there is a need in the art for chemical ligands which not only bind heavy metal ions, but also bind heavy metal ions in such a manner as to form stable, insoluble precipitates which remain stable over a range of environmental conditions and over extended periods of time.
In accordance with the purposes of the present invention as described herein, novel sulfur-containing chelate ligands are disclosed which bind heavy metals resulting in stable ligand-metal precipitates. The ligands of the present invention are suitable for binding any metal which is in or is capable of being placed in a positive oxidation state, including cadmium, iron, lead, nickel, zinc, mercury, copper, and the like. Additionally, methods for removal of heavy metals from various substances are disclosed, comprising separating selected heavy metals from selected substances by contacting said substances with an effective amount of said novel sulfur-containing chelate ligands for a sufficient time to form stable ligand-metal precipitates. The chelate ligands described herein may be used alone or in varying combinations to achieve the objects of the present invention. The ligand-metal precipitates formed by the ligands of the present invention are stable at a range of pH values from about 0 to about 14.
In one aspect, the present invention relates to chelate ligands consisting of a ring structure from which depend multiple alkyl chains terminating in sulfur-containing groups. The chelate ligands are of the general formula: 
where n is an integer from 1-4, and X is any element selected from the elements contained in Group 1a of the Periodic Table of the Elements, i.e. H, Li, Na, K, Rb, Cs, or Fr. In one useful embodiment, the chelate ligands are of the formula: 
where n is an integer from 1-4. In another useful embodiment, the chelate ligands are of the formula: 
where n is an integer from 1-4.
The chelate ligand may be a pyridine-based ligand of the formula C11H15N3O2S2, with pendant alkyl-thiol chains located at the 2,6 positions. In another particular embodiment, the chelate ligand may be a benzene-based compound of the formula C12H16N2O2S2, with pendant alkyl-thiol chains located at the 1,3 positions. In yet other embodiments, the H group of the terminal thiol groups of the above benzene- and pyridine-based compounds may be replaced with a terminal Na to form the metallated product. While not wishing to be bound by any particular theory, it is believed that the stability of the metal complexes formed by use of the compounds of the present invention derive from multiple interactions between the metal and the sulfur and nitrogen atoms on the ligand, forming a multidentate bonding arrangement around a central metal atom.
In another aspect, the present invention relates to a method of removing metal ions from a starting material. The method of the present invention comprises contacting the starting material of choice with an effective amount of a novel sulfur-containing chelate ligand as described above for a sufficient time to form a stable ligand-metal complex precipitate, said metal remaining essentially irreversibly bound to said ligand at a range of pH values from about 0 to about 14.
In yet another aspect, the present invention relates to a method of treating water (e.g. surface, ground, or waste water) containing soft heavy metals, comprising admixing said water with an effective amount of the sulfur-containing chelate ligand as described above for a sufficient time to form a stable, irreversible ligand-metal complex precipitate, and separating said precipitate from said water.
In still another aspect, the present invention relates to a method of treating aqueous acid mine drainage or water from actual mining processes which contains soft heavy metals, comprising admixing said acid mine drainage or water from mining processes with an effective amount of the sulfur-containing chelate ligand as described above for a sufficient time to form a stable, irreversible ligand-metal complex precipitate, and separating said precipitate from said acid mine drainage.
In still another aspect, the present invention relates to a method of removing soft heavy metals from soils, comprising admixing said soils with an effective amount of the sulfur-containing chelate ligand as described above for a sufficient time to form a stable, irreversible ligand-metal complex precipitate. The soils so treated may be left in situ or removed for disposal without concerns regarding leaching of said metals into the environment.
Other objects of the present invention will become apparent to those skilled in this art from the following description wherein there is shown and described a preferred embodiment of this invention, simply by way of illustration of the modes currently best suited to carry out the invention. As it will be realized, the invention is capable of other different embodiments and its several details are capable of modification in various, obvious aspects all without departing from the invention. Accordingly, the drawings and descriptions will be regarded as illustrative in nature and not as restrictive.